The present invention relates in general to imparting residual stresses to steel machine components and more particularly to a process for imparting such stresses by momentarily melting a high speed steel along a surface of the machine component.
It is well known that the stress state on or below a bearing race subjected to alternating or cyclic contact loads can have a major influence on the service life of the race. Many tapered roller bearings are made from carburizing grades of steels. When components made from these grades are carburized and heat treated, certain advantages are realized that make the components superior to through hardened components, namely more reliable, lower warranty costs, fewer quench cracks in heat treating, decreased sensitivity to grinding injury, and improved toughness or resistance to catastrophic failure.
For carburized components, compressive residual stresses develop during heat treating, and the advantages are to a large measure attributable to these residual stresses. In this regard, the absorption of carbon into a component during carburizing creates a carbon gradient. The carbon level is high near the surface and decreases as the distance away from the surface increases. When steel components are quenched from the austenitizing temperature, martensite is formed. The transformation of austenite to martensite is accompanied by an expansion in volume. The volumetric expansion is directly proportional to the carbon content of the alloy. When quenched, the surface of a component cools more rapidly than the inner portion of a component. In addition, the Ms temperature (temperature at which austenite transforms to martensite) decreases with increasing carbon content. Thus, for a carburized component, the case transforms to martensite at a lower temperature than would occur for a component corresponding in composition to the core. These two effects, operating in unison, cause a relatively high compressive residual stress to be formed at the surface layer. This effect does not occur in components having a uniform composition.
Another instance where compressive residual surface stresses are beneficial involves the press fitting of bearing components onto shafts. It is well known that this can create a tensile stress in the press-fitted component. It has been demonstrated that the press fitting of through hardened AISI 52100 steel definitely has an adverse effect on fatigue life. However, similarly press-fit bearings fabricated from carburized AISI 8620 were found to perform satisfactorily. It was concluded that under press-fitting conditions, carburized 8620 had superior fatigue characteristics compared to 52100.
For comparative purposes, typical residual stress patterns in a carburized LM 12749 cone and a through hardened LM 12749 cone manufactured from a 46100 Powered Metal steel preform are listed below:
Carburized LM 12749 Cone
A group of cones (inner races for tapered roller bearings) were rough machined from 8119 alloy steel, which is available from The Timken Company of Canton, Ohio. The steel has the following composition by weight:
______________________________________ Carbon 0.19% Molybdenum 0.10% Manganese 0.80% Nickel 0.30% Silicon 0.20% Chromium 0.40% Iron remainder ______________________________________
Each cone was carburized and then oil quenched. The cones were then rehardened by austenitizing at 1525.degree. F. for 60 minutes and then quenched into oil. The cones were then tempered at 350.degree. F. and afterwards ground to final dimensions by standard practice. They met the dimensional specifications for LM 12749 cones sold by The Timken Company of Canton, Ohio.
Residual stress measurements were made on and below the surface of the standard carburized cones by x-ray diffraction. The results were as follows:
______________________________________ Carburized 8119 LM 12749 Cones Depth Below the Surface (in.). Residual Stress (ksi) ______________________________________ 0.000 -48.1 0.005 -32.1 0.010 -28.1 0.015 -31.1 0.020 -26.6 0.025 -34.3 0.030 -22.6 0.040 -7.4 ______________________________________ Note: negative stress is compression
1% Carbon LM 12749 Cone
Cone preforms were formed using oil atomized 4600 metal powder which has the following composition after blending with carbon,
______________________________________ Carbon 1.00% Molybdenum 0.50% Manganese 0.020% Nickel 1.80% Iron remainder ______________________________________
After the preforms were compacted, sintered and hot forged, they were rough machined into green LM12749 cones. The cones were hardened by austenitizing at 1525.degree. F. for 60 minutes and then quenched into oil. The cones were then tempered at 350.degree. F. for two hours. The cones were ground to final dimensions by standard practices and met the dimensional specifications for LM12749 cones sold by The Timken Company of Canton, Ohio.
Residual stress measurements were made on and below the surface of the cones by x-ray diffraction. The results were as follows:
______________________________________ Hardened 46100 LM 12749 Cone Depth Below the Surface (in.). Residual Stress (ksi) ______________________________________ 0.000 -3.6 0.005 -2.8 0.010 -0.5 0.015 2.8 0.020 -1.2 0.025 -3.8 0.030 -3.1 0.040 0.1 ______________________________________ Note: negative stress is compression
During the welding of steel volume changes in the deposited weld metal and the base metal can occur. These volume changes can lead to distortion, very high residual stresses and often cracking in or adjacent to the weldment. They are attributable to thermal expansion on heating and thermal contraction on cooling, and phase transformations that occur in the weld metal and the heat affected zone.
When welding steel where the heated metal is attached to surrounding cold metal, the cold metal produces a resistance to dimensional change. Since the steel has a low tensile stress at elevated temperatures, most dimensional changes have to occur within the weld metal. This leads to plastic flow in the weld. If the component being welded is relatively large or rigid, residual stress is often tensile and can be as high as the yield stress of the steel. This can often lead to cracking of the weld or base material. Hence, in most steel welding operations, preheating and post welding heating is required to reduce or minimize the detrimental residual stresses that are created. Thus, conventional wisdom strongly suggests relieving residual stresses.
The present invention in contrast to conventional wisdom seeks to impart residual compressive stresses to steel machine components. This is achieved by subjecting the machine component to high energy radiation, such as a laser beam, of sufficient intensity to momentarily melt the steel. Either the steel of the original component is melted to provide a glaze or a filler metal is melted over the original component to provide a cladding. This invention is unique in that stress relief and/or full annealing of the laser processed steel is not performed. In fact the thermal cycles were chosen so as to make the residual stresses that are created during the laser processing and after the thermal treatment to be beneficial to performance. This effect is opposite to almost all welding or cladding procedures where residual stresses that are created are detrimental to performance.